This invention relates to an apparatus for deicing an antenna reflector.
During winter season, snow and ice often accumulate on the dish reflectors of satellite earth stations and microwave antennas. Since water is a dielectric, accumulation of ice and snow reduce the performance of these antenna dishes. Antenna dishes that use fixed low power levels and high frequencies signals with short wavelengths, for example satellite communication antennas, are particularly susceptible to the problems of ice and snow accumulation.
Various ice resistant and ablative chemical coatings have been developed to prevent ice and snow from forming and accumulating on antenna dishes. Chemical compounds, such as isopropyl alcohol, urea, ethylene, and propylene glycol, reduce moisture buildup by lowering the freezing point of water. Chemical coatings, however, have not provided adequate protection and often cause secondary problems, including environmental hazards, fires and excess cost. For maximum effectiveness, chemicals must be applied to the antenna reflector prior to the accumulation of the ice and snow. Coating tends to coalesce water into droplets, when the droplets increase to a critical size, they flow off the antenna reflector. In freezing temperatures, these droplets may freeze and aggravate the problem. The coatings are not effective at extremely low temperatures and have a limited life span.
Teflon coated glass fabrics used as a reflector covers or radomes have been developed to prevent ice from developing a strong enough bond for adhesion. This type of surface covering is effective only for a few seasons. Eventually, the sun's ultraviolet radiations causes cracks to form in the Teflon surface which allows the ice to bond to the surface.
For economic reasons, heating the reflector dish to prevent the accumulation of ice and snow represents the most effective solution. Common thermal deicing techniques employ the principles of conduction, convention and infrared radiation. The cost and reliability of each technique depends upon the design of the antenna reflector, including support members, and skill of designers. Convection deicing employs either electric heaters or a furnace burning gas, LPG, propane, etc. Convection heating theoretically heats the reflector uniformly. This prevents thermal stress distortion of the antenna reflector. Distortion alters the radiation characteristics of the antenna in an unpredictable manner. In satellite communications, the FCC's two degree satellite spacing requires a tight side-lobe specification to prevent interference with adjacent satellites. Even minor distortion can cause an antenna to exceed the maximum side-lobe specification.
In practice, convection systems seldom attain theoretical performance levels. Inadequate and poor air flow through the ducts causes temperature gradients. Convection systems also tend to be expensive and complex, since they employ high temperature heaters and blowers which require continual maintenance.
Conduction systems employ heaters bonded directly to the rear of the antenna reflector. Conduction systems generally have a low initial cost, but suffer from a short service life and objectionable temperature gradients. Conduction systems employing self-regulating heaters are usually more effective and reliable than those using constant wattage heaters. Reliable bonding of the heater to the rear surface of the antenna reflector is a major problem with conduction heaters. When the bond begins to fail, heat transfer is inhibited, which causes the heater to operate at higher temperatures. These high temperatures accelerate the bonding failures and ultimately, the heater temperature attains a level that destroys the bond completely. Self-regulating heaters compensate for these bonding failures. As the bond deteriorates, the heater temperature never reach destructive levels.
Heater failures create intolerable problems. First, locating the defective heater is an exceptionally difficult problem. After the defective heater is located, installing its replacement requires removing insulation and the defective heater. Proper heater re-bonding and insulation replacement may require the return of the entire antenna dish back to the factory.
U.S. Pat. No. 4,866,452 issued to Barma and assigned to Raychem uses an antenna dish with self-regulating infrared heaters bonded to black aluminum panels. The panels are mounted to the rear cover of the antenna dish or positioned at an intermediate point between the cover and the rear of the antenna dish. The panel emits infrared radiation of a long wavelength into the antenna reflector. The infrared radiation from the heaters is directed onto the black aluminum panels to de-ice the antenna dish.
U.S. Pat. No. 5,010,350 issued to Lipkin discloses paraboloidal reflector-type microwave antenna dish which has an infrared energy generator located on but not behind the rear side of the antenna. The infrared energy is directed unto the rear side of the antenna dish, the energy rear side of the infrared energy source is not radiated onto the rear side of the antenna dish. A polished surface on the rear (the side away from the antenna, not toward the antenna) of the infrared source directs almost all of the radiation onto the rear of the antenna dish. The polished surface is also not parabolic. Further, the infrared source is not collimated by the lens and thus does not provide parallel rays for the rear of the antenna reflector.
U.S. Pat. No. 3,173,141 issued to Bowman introduces a concentrated beam of heat energy, preferably infrared energy, into a small aperture located in the R-F shadow of the horn. The infrared energy will strike the radome surface at a small angle of incidence, and therefore effect appreciable heat transfer.